Lewis Structures
Lewis structures are representations of molecules showing all electrons, bonding and
nonbonding (bonding pairs and lone pairs of electrons)
Ex: H2
Step 1
 Find the sum of valence electrons of all atoms in the polyatomic ion or molecule.
If it is an anion, add one electron for each negative charge. If it is a cation,
subtract one electron for each positive charge.
Ex:PCl3 5 + 3 (7) = 26 Ex: CO32-
Step 2
 Number of electrons if each atom is to be happy: Atoms in our example will need
8 e (octet rule) or 2 e ( hydrogen). So, for the ex.
 Ex:PCl3 Ex: CO32-
Step 3
 Calculate number of bonds in the system: Covalent bonds are made by sharing of
e. You need 32 and you have 24. You are 8 e deficient. If you make 4 bonds (
with 2 e per bond) , you will make up the deficiency. Therefore,
# of bonds= ( e in step 2- e in step 1)/2 =(32-24)/2= 4 bonds
Step 4
 Draw the structure: The central atom is C ( usually the atom with least electro
negativity will be in the center (but not H). The oxygens surround it. Because
there are four bonds and only three atoms, there will be one double bond.
Step 5
 Double check your answer by counting total number of electrons.
Draw the Lewis structures for the following using above steps. Show work!
A.Cl2 B. CH2Cl2
3. NH3
Exceptions to octet rule:
1. Expanded Octet
Ex: ICl3 I has 10 electrons around it.
If an element, is in the third row or below, it
presumably can have d orbitals participate in
bonding.
2. Less than 8 electrons: H, He, Be, B
Resonance: When a molecule has more than one Lewis structures, it is said to have
resonance.
Ex: Ozone (O3) The electrons that form the second O-O bond in the double bond
do not always stay in one location but move among the three
oxygen. (They are delocalized).
Ex: Benzene, (C6H6) Has two resonance structures and is commonly depicted as a
hexagon with a circle inside it to signify the delocalized electrons
in the ring.
Ex: SCN-
Lewis Structures: Practice Problems Chemistry Honors
Write the Lewis Structures for the following molecules.
a.
ClO4-
b. SOCl2
c. PCl5
d.
NO3-
e. ICl3